The small intestine is able to buffer changes in interstitial volume following changes in capillary pressure. Interstitial glycosaminoglycans (high molecular weight polysaccharides) could play an important role in effecting this new steady-state. These large molecules of the interstitial gel phase imbibe water and partially exclude protein in proportion to molecular weight. Separation of water and protein increases osmotic activity which could be an effective buffering mechanism for controlling transcapillary transport. Excluded volume, the interstitial volume in which a molecule is not equilibrated, can be determined by analyzing lymph efflux characteristics of tagged tracers of graded molecular size. Excluded volume of each molecule should change in proportion to the amount of gel phase or decrease as the capillary filtered load of water or protein is increased. Knowing how exclusion characteristics change in vivo, the direct effect of interstitial exclusion on capillary transport will then be determined by measuring changes in interstitial water and protein. The integrated difference between arterial and venous blood flows, and the osmotic assessment of capillary water transport will provide a measure of interstitial water accumulation. Tissue monitoring of 125I-albumin will measure interstitial accumulation of protein. As the status of the gel phase changes osmotic buffering will be manifest as a change in the quantity of water and protein that equilibrate with the gel phase.